Shifting the phase of signals, such as radio frequency (RF) signals used in modern communication systems, is often used in various signal processing techniques. For example, signals provided to or from individual elements of an antenna array may each be phase shifted to provide desired phase relationships for beam forming. Likewise, signals provided to or from individual elements of an antenna array may be phase shifted to provide phase relationships for beam tilting (e.g., downward tilt for cellular telecommunications coverage).
For example, a cellular base station antenna array may employ electrical down-tilt, as may be provided through the use of appropriate phase shifting, to provide communication coverage within a designated portion of a cellular provider's service area while minimizing interference introduced with respect to neighboring portions of the service area. FIG. 1A shows antenna array 100 of a cellular base station disposed upon supporting structure (shown here as building 130) for illuminating a portion of a cellular provider's service area with RF signals to provide communication services with respect to various terminals (e.g., user equipment 140). Antenna array 100 may comprise a multi-beam array (e.g., a phased array panel, a multiple antenna system, etc.) providing various antenna beams (shown here as antenna beams 101, 102, 103, 104) for serving terminals and/or areas within a cell. The antenna beams provided by antenna array 100 are provided with down-tilt in order to direct the signals to the area of the cell and/or to minimize interference with other devices (e.g., neighboring base stations, terminals disposed outside of the cell, etc.).
Electrical down-tilt, as well as the beamformers used in forming the antenna beams, may be provided through the use of phase shifters in the signal paths of the signal feed network coupling the base station equipment to the antenna array. For example, antenna array 100 may comprise, as depicted in FIG. 1B, an antenna column 110 comprising antenna elements 111, 112, 113, 114 cooperating to provide antenna beam 101. Phase shifters 121, 122, 123, 124 of signal feed network 120 provide a signal phase progression to the signals of each of antenna elements 111, 112, 113, 114 to introduce a desired angle of down-tilt (θ, wherein θ is typically between 0° and 12° below horizontal) with respect to antenna beam 101 formed using antenna elements 111, 112, 113, 114 of antenna column 110. Where phase shifters 121, 122, 123, 124 are adjustable, the angle of down-tilt (θ) may be changed by a corresponding change in the relative phases of the signals for each of the antenna elements.
Providing signal phase shifting in some systems may present power handling and other issues. For example, the signals provided to cellular base station antenna arrays (e.g., cellular base stations operating in accordance with 3G, 4G, Long Term Evolution (LTE), etc., communication standards and protocols) may be on the order of 20 to 100 Watts of power. Many electronic components, such as diodes and surface mount phase shifters, are not suited for signals at such power levels (e.g., the components may experience excessive heating, be destroyed, provide unacceptable signal distortion, etc.). Accordingly, mechanical phase sifters (e.g., are often used for providing signal phase shifting in systems such as the above mentioned cellular communication systems.
Mechanical phase shifter configurations include wiper configurations (see e.g., U.S. Pat. Nos. 7,170,466, 7,233,217, 7,463,190, and 7,557,675, the disclosures of which are hereby incorporated herein by reference), such as may comprise a circularly rotated ceramic or thermo-magnetic material in which dielectric properties are altered to provide an adjustable matching network adjusting the phase of signals passed through the wiper phase shifter configuration. Mechanical phase shifter configurations also include ferrite configurations (see e.g., U.S. Pat. No. 3,838,363, and United States patent application publication numbers 2002/0089394 and 2010/0073105, the disclosures of which are hereby incorporated herein by reference), such as may comprise ferrite cores or blocks disposed in relationship to a microstrip line meander signal path to provide a gyromagnetic body whereby variation of the magnetization of the ferrite material provides a phase shift of signals passed through the ferrite phase shifter configuration. The foregoing mechanical phase shifter configurations tend to be quite lossy. Accordingly, a significant amount of the signal power is lost in the phase shifters.
Another mechanical phase shifter configuration which has been proposed comprises a slotted configuration (see e.g., United States patent application publication number 2009/0108957 and “A Low-Cost Electrical Beam Tilting Base Station Antennas for Wireless Communication System,” IEEE Transactions Antennas and Propagation, Vol. 52, No. 1, January 2004, the disclosures of which are hereby incorporated herein by reference), such as may comprise a slotted ground plane and perturbed metal plate used in association with a microstrip line signal path. The metal plate provides efficient signal coupling of the signal transmitted by the microstrip line signal path with the slots. Phase shifting of signals passed through the slotted configuration is provided in correspondence with the capacitance and inductance of the slots, whereby movement of the metal plate alters the coupling of the signal with the slots and thus the resulting phase shift. The foregoing slotted phase shifter configuration results in impedance matching issues. For example, as the metal plate is moved to adjust the phase shift the matching coefficient of the slotted phase shifter configuration also changes. Thus, although a matching network may be used to couple the slotted phase shifter to other circuitry (e.g., base station equipment), it becomes problematic to provide proper impedance matching over a range of phase shifts. As the impedance mismatch becomes greater, the amount of signal reflected by the slotted phase shifter configuration increases. Thus, such slotted phase shifter configurations can result in appreciable amounts of signal energy not being passed by the slotted phase shifter, thereby providing less than desirable performance.